Consider: An existential threat is one that threatens the very existence of mankind; we are at least talking about the potential to undermine the very viability of human civilization here. I don't know about you, but I find that to be ridiculous. Tell me, which major coastal city is underwater yet or about to be? The anthropocene started around 1950 according to some, so where is the devastation from the last 70 years of AGW? Where's the proof that the current heat wave in this country or climate disasters elsewhere are attributable to AGW? Could it be a factor? Yeah, maybe. But how much of a factor? Nobody really knows.
Anthropogenic global warming could be a serious problem and its impacts could be substantial—but in no way does it seriously threaten our species or human civilization. And with reasonable mitigation and adaptation, mankind will continue to move forward—reducing poverty, living healthier lives, and stabilizing our population. To claim orherwise is a lie, perpetrated by those with something to gain from our fear. It's one thing to say we might have a problem here, but it's another to say we're all gonna die of we don't spend trillions of dollars to fight AGW.